This invention relates generally to computed tomography (CT) imaging and, more particularly, to reconstructing an image from CT scan data.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d. which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelical scanxe2x80x9d is performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice are reconstructed.
Although fan-beam data may be acquired more efficiently than parallel beam data, the theory of imaging reconstruction was first developed for parallel data. In multislice fan-beam CT scanners, four variables are key in describing projection sampling. They are source angle xcex2 (and the source angle increment between two views, xcex94xcex2), fan angle xcex3 (and the fan angle increment between two rays, xcex94xcex3), individual slice aperture xcex94Zs, and pitch p=xcex94Zr/xcex94Zs, relating a table advance xcex94Zr per 360xc2x0 rotation to xcex94Zs. In practical scanning, often variable increments xcex94xcex2, xcex94xcex3, xcex94Zs, and p are determined to optimize a scanning parameter, for example, coverage in a given amount of time, and are not well matched, or more specifically, sampling is not well balanced. In the coverage optimized case, xcex94xcex2 greater than  greater than xcex94xcex3, and azimuthal resolution does not match radial resolution. Decreasing view sampling time to reduce xcex94xcex2 requires very expensive and pervasive system design changes, including changes to the data acquisition system (DAS), slip ring and reconstruction system. In other situations, it is possible to increase scanning time, for example, in head imaging, where patient immobilization is easy to achieve. It is then possible to obtain xcex94xcex2 less than xcex94xcex3, while directly reducing xcex94xcex3 would require expensive and complex system design changes, such as reducing detector cell pitch and increasing a number of detector cells, or enabling focal spot wobbling. Moreover, in helical scanning, the quality of the reconstructed image is affected by pitch size, that is, the distance through which the object is moved relative to the z-axis as the gantry rotates 360 degrees. Frequently, to reduce total scan time, either pitch size or slice collimation is increased. As a consequence, the quality of the reconstructed image is reduced.
Rebinning data from fan-beam to parallel would fully leverage all data in a scan, or of a portion of a scan. For instance, two or more xe2x80x9cconjugate raysxe2x80x9d that provide a measurement of a given line integral in an axial scan comprising 360xc2x0 or more of projection data could be combined to effectively decrease xcex94xcex3 by a factor 2 to xcex94xcex3/2 via what is known in the art as xe2x80x9cquarter offset.xe2x80x9d Further, as a given, rebinned, i.e., not measured directly, Radon space sample corresponds to a line integral through a patient, estimation of such Radon space samples in theory requires knowledge of all scan projection data, or of portions of scan projection data sufficient to reconstruct an image though a slice of interest. This requirement comes about because reconstruction of a single pixel value involves all such projection data. Accordingly, rebinning the data from fan-beam to parallel provides leveraging of a higher resolution available along one sample direction to increase an effective resolution along another sampling direction, and therefore remedies scan acquisition sampling limitations along the latter direction. Although this leveraging is described below in terms of partial differential equations, it can as well be understood from spectral considerations for a band-limited or quasi-band-limited function, such as a slice of interest in computed tomography. Indeed, the sampling theorem indicates that, providing a 2D or 3D sampling density is sufficient, undersampling in one direction can be compensated by finer sampling along the other directions. Gridding methods may be effectively applied to regrid the samples on a uniformly sampled set of estimates. Similarly, it is well known in the art that sampling a function and its first derivative simultaneously effectively allows doubling of a sample interval and recovery of an original function.
It would therefore be desirable to provide a method for rebinning fan data into parallel data for improved image reconstruction. It would also be desirable to obtain and utilize additional information in each scan to increase the quality of the reconstructed images. In addition, it would be desirable to provide methods for obtaining such additional information from nonuniformly sampled projection data. It also would be desirable to provide a method for increasing resolution that provides for increased helical scanning coverage in a given amount of time without degradation of image quality.
There is thus provided, in one embodiment of the present invention, a method for producing an enhanced tomographic image of an object. The method includes steps of obtaining fan beam projection data of the object from a tomographic scan; rebinning the fan beam projection data into a quantity of parallel projection data points; applying interpolation to the quantity of parallel projection data points to increase the quantity of parallel projection data points; and generating a tomographic image from the increased quantity of parallel projection data points.
The methods and apparatus of the present invention provide rebinning fan data into parallel data for improved image reconstruction. Additional information is obtained and utilized in each scan to increase the quality of the reconstructed images. The additional information can be obtained and used even in the case of nonuniformly sampled projection data. The methods and apparatus of the present invention also provide increased helical speed coverage by using thicker slice collimation, without degradation of image quality, and increased volume coverage while maintaining z-resolution in a reconstructed slice.